The present invention relates to cables, in particular for medium-voltage or high-voltage electrical power transmission or distribution, for telecommunications or for data transmission, as well as combined power/telecommunications cables, in which at least one coating layer consists of a halogen-free recyclable material which has high-performance mechanical and electrical properties. There is currently a great need for products which are highly environment-friendly, formed of materials which are not only harmless to the environment both during production and during use, but are also easy to recycle at the end of their life. However, the use of environment-friendly materials is highly conditioned by the requirements for keeping their costs within acceptable limits and at the same time ensuring satisfactory performance levels under the most common conditions of use, or even better performance than those of conventional materials. in the cables sector, in particular medium-voltage or high-voltage electrical power transmission or distribution cables, the various coatings which surround the conductor usually consist of crosslinked polymer material, in particular of polyethylene or ethylene copolymers suitably crosslinked during the extrusion phase. The reason for this is that these crosslinked materials maintain a high degree of flexibility and satisfactory mechanical properties even under hot conditions in continuous use and/or under conditions of current overload. However, it is well known that crosslinked materials are not recyclable and that, at the end of their life, they can only be disposed of by incineration. In addition, in certain cases the outer protective sheath is formed of polyvinyl chloride (PVC) which is difficulty to separate by conventional methods (for example by density differences in water)xe2x80x94from crosslinked polyolefins containing inorganic fillers (for example ethylene/propylene rubbers containing inorganic fillers), nor can it be incinerated just as it is, by combustion, since this generates highly toxic chlorinated products.
Patent application WO 96/23311 describes a low-voltage high-current cable in which the insulating coating, the inner sheath and the outer sheath are made of the same non-crosslinked polymer material which is coloured black by the addition of carbon black. Using the same material would not require the separation of the abovementioned components in a recycling process. When temperatures below 70xc2x0 C. are used, a polyethylene with a density of between 0.92 and 0.94 g/cm2 and a Shore D hardness xe2x89xa742 is proposed as polymer material for the insulating coating, the inner sheath and the outer sheath. The use of thermoplastic elastomers consisting of two-phase mixtures of polypropylene with an ethylene/propylene co- or terpolymer (EPR or EPDM rubber) is proposed in the case of a maximum working temperature of 90xc2x0 C. Within the latter class of polymers, mention is specifically made of the commercial products Santoprene(copyright) from Monsanto (polypropylene-based thermoplastic elastomer) and Novolen(copyright) from BASF (heterogeneous propylene copolymers obtained in a reactor and having an ethylene/propylene elastomeric phase content of greater than 25% by weight, for example 43% by weight of ethylene/propylene rubber, such as Novolen(copyright) 2912 HX from BASF). The Applicant has observed that the technical problem of obtaining a recyclable polymer material for coating electrical cables, in particular medium- or high-voltage cables, which has the desired combination of electrical and mechanical properties can be solved by using a heterogeneous copolymer comprising an ethylene-based elastomeric phase copolymerized with an xcex1-olefin and a propylene-based thermoplastic phase, characterized in that the elastomeric phase is at least 45% by weight relative to the total weight of the heterogeneous copolymer and in that this copolymer is essentially free of crystallinity deriving from polyethylene sequences. In point of fact, it has been suggested that a large amount of elastomeric phase combined with a substantial absence of crystalline polyethylenic sequences gives, on the one hand, the desired mechanical properties for an electrical cable, and in particular a breaking load value of greater than 12 MPa (measured according to CEI standard 20-34 xc2xa7 5.1), and, on the other hand, excellent electrical insulation properties, in particular low dielectric losses, even under hot conditions and in particular at the maximum working temperature of the cable, with values for the tangent of the angle of loss (tan delta) at 90xc2x0 C. (measured according to ASTM standard D150) of less than 5xc3x97104. In a first aspect thereof, the present invention thus relates to a cable comprising a conductor and at least one coating layer based on a non-crosslinked polymer material comprising a heterogeneous copolymer with an ethylene-based elastomeric phase copolymerized with an xcex1-olefin and a propylene-based thermoplastic phase, characterized in that the said elastomeric phase in the said heterogeneous copolymer is at least 45% by weight relative to the total weight of the heterogeneous copolymer, and in that the said heterogeneous copolymer is essentially free of crystallinity deriving from polyethylene sequences.
According to another aspect, the invention relates to a cable comprising a conductor and at least one coating layer, in which the said coating layer has electrical insulating properties and is based on a non-crosslinked polymer material comprising a heterogeneous copolymer as defined above.
According to a further aspect, the invention relates to a cable comprising a conductor and at least one coating layer, in which the said coating layer has semiconductive properties and is based on a non-crosslinked polymer material comprising a heterogeneous copolymer as defined above.
According to a further aspect, the invention relates to a cable comprising a conductor and at least one coating layer, in which the said coating layer functions as an outer protective sheath and is based on a non-crosslinked polymer material comprising a heterogeneous copolymer as defined above.
According to a further aspect, the invention relates to a cable comprising a conductor and at least one coating layer based on a non-crosslinked polymer material, in which at least 70%, preferably at least 90%, by weight of the said non-crosslinked polymer material consists of a heterogeneous copolymer as defined above.
For the purposes of the present description and the claims which follow, the expression xe2x80x9cheterogeneous copolymer comprising an ethylene-based elastomeric phase copolymerized with an xcex1-olefin and a propylene-based thermoplastic phasexe2x80x9d means a thermoplastic elastomer obtained by sequential copolymerization of: (a) propylene, optionally containing small amounts of at least one olefinic comonomer chosen from ethylene and xcex1-olefins other than propylene; and then: (b) a mixture of ethylene with an xcex1-olefin, in particular propylene, and optionally with small proportions of a diene. This class of products is also commonly known as xe2x80x9cthermoplastic reactor elastomersxe2x80x9d.
Throughout the present description and the claims which follow, the expression xe2x80x9cheterogeneous copolymer essentially free of crystallinity deriving from polyethylenic sequencesxe2x80x9d means that the heterogeneous copolymer subjected to differential scanning calorimetry (DSC) analysis does not show any appreciable melting peaks attributable to a crystalline polyethylenic phase, i.e. to (CH2)n sequences of crystalline type. In quantitative terms, this means that the value of the enthalpy of fusion of peaks present below 130xc2x0 C. and attributable to polyethylenic sequences is generally less than 3 J/g: preferably, it is substantially zero.
Alternatively, the substantial absence of crystallinity due to polyethylenic sequences can be ascertained by extraction of the elastomeric (amorphous) phase by means of a suitable organic solvent (for example xylene at 135xc2x0 C. at reflux for 20 min.) and analysis of the residue formed by the crystalline phase, for example by means of X-ray diffractometry. The substantial absence of reflection typical of crystalline polyethylene at the angle 2xcex8=21.5xc2x0 (with radiation of the copper) indicates that the heterogeneous copolymer is essentially free of crystalline polethylenic sequences.
The amount of elastomeric phase present in the heterogeneous copolymer can be determined by known techniques, for example by extracting the elastomeric (amorphous) phase with a suitable organic solvent (in particular xylene at 135xc2x0 C. at reflux for 20 min): the amount of elastomeric phase is calculated as the difference between the initial weight of the sample and the weight of the dried residue.
In accordance with the present invention, the use of a heterogeneous copolymer as defined above gives a flexible recyclable coating with excellent mechanical properties, in terms of both breaking load and elongation at break. In particular, it is possible to obtain mechanical performance levels under hot conditions, i.e. at 90xc2x0 C. for continuous use and at 130xc2x0 C. in the case of a current overload, which are comparable with the typical performance levels of the polyethylene-based crosslinked coatings currently marketed, thereby making the abovementioned heterogeneous copolymer suitable for coating not only low-voltage electrical cables, but preferably medium-voltage or high-voltage cables.
For the purposes of the present invention, the term xe2x80x9clow voltagexe2x80x9d generally means a voltage of less than 5 kV, the term xe2x80x9cmedium voltagexe2x80x9d means a voltage of between 5 and 35 kV, while xe2x80x9chigh voltagesxe2x80x9d are considered to be voltages above 35 kV.
With particular reference to medium-voltage and high-voltage cables, the heterogeneous copolymers as defined above can be used advantageously to prepare an insulating layer. The reason for this is that, as mentioned above, these copolymers have high-performance mechanical properties both at room temperature and under hot conditions, and moreover have adequate electrical properties, with low tan delta and permittivity values and thus low dielectric losses in alternating current, which, as is known, are proportional to the product between tan delta and permittivity.
In addition, the same heterogeneous copolymers defined above can be used advantageously to prepare at least one inner or outer semiconductive layer. The reason for this is that the addition to these copolymers of fillers capable of imparting semiconductive properties, for example carbon black, does not substantially challenge the mechanical properties, which are maintained well below the values considered as suitable for semiconductive layers. The possibility of using the same type of polymer material both for the insulating layer and for the semiconductive layers is particularly advantageous in the production of medium-voltage or high-voltage cables, since it ensures optimal adhesion between adjacent layers and thus better electrical behaviour, especially at the interface between the insulating layer and the inner semiconductive layer where the electrical field is stronger and thus the risk of partial discharges are greater.
Heterogeneous copolymers whose structural properties and relevant physicochemical properties vary within a wide range are commercially available, for example under the tradename Hifax(copyright) from Montell. However, by means of the teaching provided in the present description, a person skilled in the art will readily be able to select the heterogeneous copolymers which are most suitable for carrying out the present invention.
Generally, the said heterogeneous copolymers are prepared by sequential copolymerization of: (a) propylene, optionally containing at least one olefinic comonomer chosen from ethylene and xcex1-olefins other than propylene; and then of: (b) a mixture of ethylene with an xcex1-olefin, in particular propylene, and optionally a diene. The copolymerization is usually carried out in the presence of Ziegler-Natta catalysts based on halogenated titanium compounds supported on magnesium chloride. Details regarding the preparation of these copolymers are given, for example, in EP-A-0,400,333, EP-A-0,373,660 and U.S. Pat. No. 5,286,564.
The term xe2x80x9cxcex1-olefinxe2x80x9d refers to an olefin of formula CH2=CHxe2x80x94R, where R is a linear or branched alkyl containing from 1 to 10 carbon atoms. The said xcex1-olefin can be chosen, for example, from: propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-dodecene and the like.
The thermoplastic phase of the heterogeneous copolymer, mainly produced during the abovementioned phase (a) of the process, consists of a propylene homopolymer or a crystalline copolymer of propylene with an olefinic comonomer chosen from ethylene and xcex1-olefins other than propylene. Preferably, the olefinic comonomer is ethylene. The amount of olefinic comonomer is preferably less than 10 mol % relative to the total number of moles of the thermoplastic phase.
As mentioned above, the elastomeric phase of the heterogeneous copolymer, mainly produced during the abovementioned phase (b) of the process, is at least 45% by weight, preferably at least 55% by weight and even more preferably at least 60% by weight, relative to the total weight of the heterogeneous copolymer, and consists of an elastomeric copolymer of ethylene with an xcex1-olefin and optionally with a diene. The said cc-olefin is preferably propylene. The diene optionally present as comonomer generally contains from 4 to 20 carbon atoms and is preferably chosen from: linear (non-)conjugated diolefins, for example 1,3-butadiene, 1,4-hexadiene, 1,6-octadiene and the like; monocyclic or polycyclic dienes, for example 1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene and the like. The composition of the elastomeric phase is generally as follows: from 15 to 85 mol % of ethylene, from 15 to 85 mol % of xcex1-olefin, from 0 to 5 mol % of a diene.
In a preferred embodiment, the elastomeric phase consists of an elastomeric copolymer of ethylene and propylene which is rich in propylene units, this copolymer in particular having the following composition: from 15 to 50% by weight, more preferably from 20 to 40% by weight, of ethylene, and from 50 to 85% by weight, more preferably from 60 to 80% by weight, of propylene, relative to the weight of the elastomeric phase.
The amount of propylene units in the elastomeric phase can be determined by extraction of the elastomeric phase as described above (for example with xylene at 135xc2x0 C. at reflux for 20 min), followed by analysis of the dried extract according to known techniques, for example by infrared (IR) spectroscopy.
In one embodiment of the present invention, the said at least one coating layer based on a non-crosslinked polymer material comprises a mixture of a heterogeneous copolymer as defined above with a thermoplastic polymer which has a melting point of greater than 160xc2x0 C. This thermoplastic polymer is preferably chosen from crystalline propylene homopolymers and copolymers with a enthalpy of fusion of greater than 75 J/g, preferably greater than 85 J/g. The presence of this thermoplastic polymer makes it possible to increase the heat-pressure resistance of the cable coating, and is particularly preferred when the thermoplastic phase of the heterogeneous copolymer used has a melting point of less than 150xc2x0 C. The amount of thermoplastic polymer to be used mixed with the heterogeneous copolymer according to the present invention is generally between 10 and 50% by weight, preferably between 20 and 40% by weight, relative to the total weight of the said mixture.
Other conventional components such as antioxidants, fillers, processing co-adjuvants, lubricants, pigments, water-free retardant additives, voltage stabilizer additives and the like can be added to the base polymer material consisting of a heterogeneous copolymer as defined above, optionally mixed with the said thermoplastic polymer. When it is intended to prepare a semiconductive layer, the polymer material is preferably filled with carbon black, in an amount such as to give this material semiconductive properties (i.e. so as to obtain a resistivity of less than 5 ohm.m at room temperature). This amount is generally between 5 and 80% by weight, preferably between 10 and 50% by weight, relative to the total weight of the compound.
Conventional antioxidants which are suitable for this purpose are, for example: polymerized trimethyidihydroquinoline, 4,4xe2x80x2-thiobis(3-methyl-6-tert-butyl)phenol; pentaerithrityl tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,2xe2x80x2-thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the like, or mixtures thereof.
Other fillers which can be used in the present invention include, for example: calcium carbonate, calcined kaolin, talc and the like, or mixtures thereof. Processing co-adjuvants usually added to the polymer base are, for example, calcium stearate, zinc stearate, stearic acid, paraffin wax, silicone rubbers and the like, or mixtures thereof.